Video Preview window size

I have a project 1280x720 square pixel, preview window is on secondary monitor, double clicking the title bar toggles the display size between 591x333 and 1232x693. Can I change a setting somewhere so that my display toggles between 640x360 and 1280x720? Monitors are set at 1920x1200, which is the recommended setting and highest resolution on my Dell 24's. As usual, probably a simple switch somewhere but I can't find it.

thx
Steven